In the field of LED drivers for offline applications such as retrofit lamps, solutions are demanded to cope with high efficiency, high power density, long lifetime, high power factor and low cost, among other relevant features. While practically all existing solutions compromise one or the other requirement, it is essential that the proposed driver circuits properly condition the form of the mains power into the form required by the LEDs while keeping compliance with present and future power mains regulations. Of critical importance is to guarantee a maximum light perceptible flicker (preferably zero) at the same time that the power factor is maintained above a certain limit.
Further, in off-line converters, energy from the power mains often needs to be synchronously drawn in proportion to the supplied voltage waveform in order to achieve high power factor and low harmonic distortion. Power converter architectures with an independent preconditioner stage are traditionally employed to best accomplish this task without compromising the proper form of the energy to be delivered to the load.
Typically, two series connected power stages are employed to obtain high power factor while keeping the output power constant throughout a mains cycle (or supply cycle, i.e. the cycle of the mains voltage or the supply voltage). In those architectures the first stage shapes the mains' current and the second stage performs the power conversion to the load.
Nonetheless, for reasons related to complexity and cost, simplified powertrain solutions are adopted known conventionally as single-stage, where either of the two stages may essentially not be incorporated. As a consequence of such simplification, the aforementioned requirements may be largely compromised and/or the converter performance highly degraded, particularly in terms of size, reliability and lifetime. The latter is usually mainly attributed to the need of using a bulky low frequency storage capacitor in parallel to the load when constant output power delivery is to be guaranteed.
Single stage solutions are common in literature. One reference example is given in the work of Robert Erickson and Michael Madigan, entitled “Design of a simple high-power-factor rectifier based on the flyback converter”, IEEE Proceedings of the Applied Power Electronics Conferences and Expositions, 1990, pp. 792-801.
An intermediate solution laying half-way between the two-stage and single-stage approaches is the single-stage converter with integrated preconditioner. Such solutions can feature reduced component count and high power density while keeping compliance with both load and power mains requirements. Other embodiments with a single power converting stage allow high power factors (HPF) by means of integrating a boost converter operating in discontinuous conduction mode. These converters actually combine the above mentioned two power conversion stages.
A HPF converter for compact fluorescent lamps is described in “High-Power-Factor Electronic Ballast with Constant DC-Link Voltage”, by Ricardo de Oliveira Brioschi and José Luiz F. Vieira, IEEE Transactions on Power Electronics, vol. 13, no. 6, 1998. Here, a half bridge is shared by a boost converter and an LC parallel resonant converter, which is operated above resonance in order to obtain zero voltage switching (ZVS). To further support ZVS the bus voltage is controlled constant. Such a HPF converter, however, typically requires a large bus capacitor and an output rectifier and has only narrow supply voltage and load (drive) voltage ranges.
Another example of integrated power stages is the work of R. Venkatraman, A. K. S. Bhat and Mark Edmunds, entitled “Soft-switching single-stage AC-to-DC converter with low harmonic distortion”, IEEE Transaction on Aerospace and Electronic Systems, vol.36, no.4, October 2000. In this work, a high frequency transformer isolated, Zero Voltage Switching (ZVS), single stage AC-DC converter with high power factor and low harmonic distortion is presented and analyzed for a constant power load.